In addition to exhibiting changes in stiffness, measured with indexes such as Young's modulus, tissues may undergo viscosity changes which impact the way these tissues respond to excitations of different frequencies. Known systems involve generating imaging mechanical waves in tissues in response to an external excitation. The sector of magnetic resonance imaging encompassing these systems is known as Magnetic Resonance Elastography (MRE). These known systems employ a motion encoding gradient (MEG) embedded in a rapid imaging sequence such as gradient echo or echo planar imaging. An external device capable of generating and transmitting mechanical waves into the body is synchronized in frequency and phase to the MEG. By acquiring phase images, the combination of the external wave generator and the MEG enables the visualization of mechanical shear waves as they propagate through the tissue being interrogated. Through an analytical “inversion” process, the measured mechanical wavelengths are converted into a quantitative estimate of shear modulus.
Extensions of these known systems allow the acquisition of multi-spectral information. This is accomplished by either acquiring separate MRE data at different frequencies or by utilizing bandpass filtering to generate images at different frequencies. These systems require substantial post processing and longer acquisition times to collect the data. A system according to invention principles addresses these deficiencies and related problems.